The culture process of mammalian cells, animal cells, insect cells, bacteria, yeast and molds has one major rate limiting step, oxygen mass transfer. Oxygen metabolism is essential for metabolic function. In mammalian and animal cell culture it is especially important during the early stages of rapid cell division. Oxygen utilization per cell is greatest when cells are suspended; requirements decreasing as the cells aggregate and differentiate. However, during the later phases of cell culture, as the number of cells per unit volume increases, the bulk oxygen mass transfer requirements increase. Traditionally, increased requirements are accommodated by mechanical stirring methods. However, these methods create fluid shear gradients that can damage cellular growth.
There are several basic strategies for increasing the gas mass transfer across a membrane: increase oxygen concentration; increase the rate of transfer from the air to the media; and/or increase the surface area for gas exchange. Increasing the oxygen partial pressure will increase the bulk oxygen transfer. However, a boundary layer of oxygen toxicity will form at the gas permeable membrane-media interface. Cells entering the toxic boundary layer could sustain irreparable damage.
Approaches to increasing the rate of gas transfer at the air-membrane-media interface include: increase the rate of air movement across the membrane with air pumps or other mechanical means; increase the gas diffusion rate across the membrane by selecting a more gas permeable membrane; and/or increase the rate of media flow past the membrane. In each approach, the rate of exchange across the gas permeable membrane is augmented, leading to improved gas exchange.
The third approach for increasing bulk gas transfer is to increase the air-membrane-media surface area. An enlarged surface enhances the bulk gas transfer. Improved gas exchange serves to both increase oxygen availability and remove the carbon dioxide by-product.
Each of the above-described potential means for increasing gas mass transfer in cell cultures is either detrimental to the cells in the culture medium, and/or requires substantial modification of the apparatus and/or medium employed for culture. Accordingly, there is still a need in the art for simplified, readily implemented means to improve gas mass transfer in cell culture.